The multifunctional tixw1 xo2 (x=0 5; 0 6; 0 7; 0 8) support for platinum to enhance the activity and co tolerance of direct alcohol fuel cells

VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY HAU QUOC PHAM THE MULTIFUNCTIONAL TixW1-xO2 x = 0.5; 0.6; 0.7; 0.8 SUPPORT FOR PLATINUM TO ENHAN

VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

HAU QUOC PHAM

THE MULTIFUNCTIONAL TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) SUPPORT FOR PLATINUM TO ENHANCE THE ACTIVITY AND CO-TOLERANCE OF DIRECT ALCOHOL FUEL CELLS

A dissertation submitted for the degree of

Doctor of Philosophy

HO CHI MINH CITY – 2022

VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

HAU QUOC PHAM

THE MULTIFUNCTIONAL TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) SUPPORT FOR PLATINUM TO ENHANCE THE ACTIVITY AND CO-TOLERANCE OF DIRECT ALCOHOL FUEL CELLS

A dissertation submitted for the degree of

Doctor of Philosophy

Major subject: Chemical Engineering

Major subject code: 9520301

Independent Reviewer:

Independent Reviewer:

Reviewer: ASSOC PROF DR TRAN VAN MAN

Reviewer: ASSOC PROF DR NGUYEN DINH THANH

Reviewer: ASSOC PROF DR NGUYEN NHI TRU

Advisor: 1 ASSOC PROF VAN THI THANH HO

PLEDGE

I pledge that this dissertation is my study under the direct guidance of Assoc Prof Van Thi Thanh Ho and Assoc Prof Son Truong Nguyen The study results and conclusions in this dissertation are honest, and not copied from any one source and in any form The reference to the sources of documents (if any) has been cited and the reference sources are recorded as prescribed

Signature

Hau Quoc Pham

ABSTRACT The worldwide environment has been getting worse day by day because of the emission of various harmful pollutants into the environment from burning traditional fossil fuels Also, fossil fuels are limited resources and will be exhausted in the next few decades, therefore, finding out sustainable and renewable energy sources has sparked interest as future alternatives Recently, direct alcohol fuel cells (DAFCs) have been considered a promising green energy source in portable and transportation applications due to their relatively simple infrastructure, portability, operation cost, easy storage, and conveyance of alcohol fuels Nonetheless, the sluggish oxidation kinetics and “CO-like poisoning” effect of catalysts are limitations for commercializing DAFCs Alloying Pt with Ru is regarded as an efficient anodic DAFC catalyst owing to its high electrochemical activity and great CO anti-poisoning ability The Ru metal, however, can be dissolved at the fuel cell operation potential, resulting in a decrease in the electrocatalytic stability of this alloy catalyst Furthermore, the high price and low natural abundance of Ru are drawbacks of particular uses

To address aforementioned problems, we fabricate TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanostructures as multifunctional support with co-catalytic functionality to prevent or reduce the deterioration of anodic catalyst in DAFCs Additionally, tuning morphology and structure of metal catalyst are also combined to enhance the catalytic performance

of electrocatalyst, thereby promoting large-scale DAFC applications

Various mesoporous TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) supports are fabricated by a facile solvothermal route to comprehend the effect of doping tungsten concentration on electrochemical properties of 20 wt% Pt/TixW1-xO2 catalysts for ethanol electro-oxidation reaction (EOR) As a result, the surface area and electrical conductivity of as-prepared supports are drastically increased with the doped tungsten contents of 20 at% (Ti0.8W0.2O2) and 30 at% (Ti0.7W0.3O2) With rising the doped tungsten content up to 40 at% (Ti0.6W0.4O2), the electrical conductivity is almost unchanged, whereas the surface area is remarkably decreased This implies that the addition of proper doped tungsten content into TiO2 lattices results in an enormous enhancement in both surface area and

the support surface by a rapid microwave-assisted polyol route In term of the EOR, 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) catalyst shows the catalytic performance better than the commercial 20 wt% Pt/C (E-TEK) catalyst Among as-made catalysts, 20 wt% Pt/Ti0.7W0.3O2 catalyst displays the highest mass activity (260 23 mA mgPt-1) and largest

If/Ib ratio (1.34), which are 2.0- and 1.57-fold greater than those of 20 wt% Pt/C TEK) catalyst (130.62 mA mgPt-1 for mass activity and 0.85 for If/Ib value, respectively) After 5000-cycling ADT, as-made catalysts show the mass activity loss about twice as lower than the commercial catalyst that exhibits great EOR stability Experimental results demonstrate that TixW1-xO2 supports can be utilized as a suitable alternative for the common carbon material in the function of catalyst support for fuel cells

(E-For the first time, a combination of using non-carbon nanosupport and tuning the morphology and structure of metal catalyst is utilized to assemble a robust catalyst for alcohol oxidation reaction (AOR) The one-dimensional (1D) Pt nanowires (NWs) are successfully grown on the Ti0.7W0.3O2 surface by a simple chemical reduction route at room temperature, only using formic acid as a reducing agent These observational results indicate that the 1D Pt NWs/Ti0.7W0.3O2 catalyst is a potential anodic catalyst for the oxidation reaction of methanol (MOR) and ethanol (EOR), which can replace a conventional Pt NPs/C catalyst For instance, 1D Pt NWs/Ti0.7W0.3O2 catalyst exhibits

a low onset potential (~0.1 VNHE for MOR and ~0.2 VNHE for EOR), high mass activity (355.29 mA mgPt-1 for MOR and 325.01 mA mgPt-1 for EOR), and impressive electrochemical stability compared to the Pt NPs/C catalyst The outstanding activity and stability of 1D Pt NWs/Ti0.7W0.3O2 catalyst can be interpreted due to the unique properties of 1D Pt nanostructures and advantages of Ti0.7W0.3O2, as well as synergetic effects between 1D Pt NWs and Ti0.7W0.3O2 support

More importantly, 1D-bimetallic Pt3Co NWs with a diameter of around 4 nm and several tens of nanometers in the lengths are grown on the Ti07W0.3O2 by a template- and surfactant-free chemical reduction method The 1D-bimetallic Pt3Co NWs/Ti0.7W0.3O2 catalyst exhibits high mass activity (393.29 mA mgPt-1 for MOR and 341.76 mA mgPt-1 for EOR) and great electrochemical durability compared to conventional Pt NPs/C catalyst In addition, the CO-stripping result shows superior CO-

tolerance of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst with the COads oxidation peak at 0.64

VNHE, which is much lower than that of the Pt NPs/C catalyst (0.78 VNHE) After cycling ADT, the activity loss of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst is 10.68% of the initial mass activity, which was 4.18-time lower than that of the Pt NPs/C catalyst (44.66%), indicating superior stability retention of 1D Pt3CoNWs/Ti0.7W0.3O2 catalyst These enhancements are attributable to (i) advantages of 1D nanostructures with abundant active catalytic sites facilitating the oxidation of adsorbed small organic molecules; (ii) addition of Co promotes the removal of strongly bound intermediates on

5000-Pt sites neighboring Co, resulting in boosting CO-tolerance of as-prepared catalyst; (iii) synergic and electronic effects of compounds, Pt3Co NWs, and Ti0.7W0.3O2 support This work can open up an effective approach to enhance the performance of catalysts with a decrease in Pt consumption for electrochemical energy conversion

TÓM T T LU N ÁN

Bi n đ i khí h u và ô nhi m môi tr ng trên th gi i ngày càng tr nên t i t do s phát th i c a các ch t ô nhi m t vi c đ t các nhiên li u hóa th ch truy n th ng H n

n a, tr l ng nghiên li u hóa th ch trên th gi i là gi i h n và s c n ki t trong vài

th p k t i, do đó, nhu c u tìm ki m các ngu n n ng l ng thay th s ch và có kh n ng tái t o đ c u tiên Pin nhiên li u s d ng tr c ti p alcohol (DAFCs) đang đ c nghiên

c u và s d ng trong nhi u l nh v c nh v n chuy n và các thi t b c m tay do ít phát

th i khí nhà kính, hi u su t chuy n đ i n ng l ng t ng đ i cao, chi phí v n hành th p,

kh n ng l u tr và v n chuy n d dàng và an toàn c a nhiên li u alcohol Tuy nhiên,

đ ng h c cho ph n ng oxi hóa ch m và s ng đ c CO c a xúc tác Pt là nh ng h n

ch chính nh h ng tr c ti p t i hi u su t ho t đ ng c a DAFCs trong th i gian ho t

đ ng lâu dài V t li u xúc tác h p kim Pt v i Ru đang đ c s d ng nh xúc tác hi u

qu cho ph n ng oxi hóa nhiên li u alcohol do ho t tính xúc tác và kh n ng ch ng

ng đ c CO cao, nh ng s d hòa tan c a kim lo i Ru t i th ho t đ ng c a pin nhiên

li u d n t i s không n đ nh c a v t li u xúc tác này Ngoài ra, giá thành cao và l ng

Ru t nhiên t ng đ i th p c ng là m t nh c đi m c a xúc tác Pt-Ru

gi i quy t v n đ này, tôi t ng h p và kh o sát đ c tính c a v t li u c u trúc nano

TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nh v t li u n n xúc tác đa ch c n ng v i vai trò đ ng xúc tác đ c i thi n ho t tính và đ b n c a v t li u xúc tác trong pin nhiên li u s d ng

tr c ti p alcohol Ngoài ra, vi c đi u khi n hình d ng và c u trúc c a kim lo i xúc tác

c ng đ c k t h p trong lu n án này đ c i thi n ho t tính xúc tác c a v t li u xúc tác

đi n hóa cho ph n ng oxi hóa alcohol, thúc đ y s ng d ng c a pin nhiên li u s d ng

tr c ti p alcohol

V t li u n n TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) đ c t ng h p b ng ph ng pháp dung nhi t đ kh o sát s nh h ng c a l ng Vonfram (W) pha t p lên đ c tính xúc tác

đi n hóa c a v t li u xúc tác 20 wt% Pt/TixW1-xO2 cho ph n ng oxi hóa ethanol (EOR)

K t qu ch ra r ng di n tích b m t riêng và đ d n đi n c a v t li u n n t ng đáng k khi l ng W pha t p là 20 at% (Ti0.8W0.2O2) và 30 at% (Ti0.7W0.3O2), và khi t ng lên t i

40 at% thì đ d n đi n h u nh không thay đ i, trog khi đó di n tích b m t riêng gi m

rõ r t i u này ch ng t r ng khi thêm m t l ng W pha t p tích h p d n t i s c i

thi n c di n tích b m t riêng và đ d n đi n c a v t li u n n TixW1-xO2 Bên c nh đó,

5000 vòng quét th tu n hòa, v t li u xúc tác t ng h p đ c cho th y s suy ho t tính

th p h n kho ng 2 l n so v i xúc tác th ng m i, đi u này cho th y đ b n xúc tác t t

c a v t li u t ng h p đ c Nh ng k t qu trên cho th y r ng v t li u n n TixW1-xO2 có

th là s thay th thích h p cho v t li u carbon th ng m i trong pin nhiên li u

L n đ u tiên, v t li u xúc tác 1D Pt d ng s i (nanowires) đ c t ng h p thành công trên v t li u n n Ti0.7W0.3O2 b ng ph ng pháp kh đ n gi n t i nhi t đ phòng s d ng formic acid nh ch t kh V t li u xúc tác 1D Pt NWs/Ti0.7W0.3O2 th hi n là v t li u xúc tác ti m n ng cho quá trình oxi hóa methanol (MOR) và ethanol (EOR) có th thay

th v t li u xúc tác truy n th ng Pt NPs/C C th , xúc tác 1D Pt NWs/Ti0.7W0.3O2 th

hi n th kh i phát quá trình oxi hóa nhiên li u th p (~0.1 VNHE cho MOR và ~0.2 VNHEcho EOR), c ng đ oxi hóa cao (355.29 mA mgPt-1 cho MOR và 325.01 mA mgPt-1cho EOR), c ng nh đ b n xúc tác t t so v i v t li u xúc tác Pt NPs/C S c i thi n này đ c gi i thích là do u đi m c a c u trúc nano 1D Pt và v t li u n n Ti0.7W0.3O2

c ng nh hi u ng liên h p gi a 1D Pt NWs và v t li u n n Ti0.7W0.3O2

c bi t, xúc tác h p kim 1D Pt3Co d ng s i (NWs) v i đ ng kính kho ng 4 nm và dài vài ch c nanomet c ng đ c t ng h p thành công trên v t li u n n Ti0.7W0.3O2 b ng

ph ng pháp kh đ n gi n không s d ng khung m u hay ch t ho t đ ng b m t V t

li u xúc tác 1D Pt3Co NWs/Ti0.7W0.3O2 th hi n ho t tính cao (393.29 mA mgPt-1 cho MOR và 341.76 mA mgPt-1 cho EOR), và đ b n xúc tác t t so v i v t li u xúc tác truy n th ng Pt NPs/C Ngoài ra, ph ng pháp CO-stripping th hi n kh n ng ch ng

ng đ c CO v t tr i c a xúc tác 1D Pt3Co NWs/Ti0.7W0.3O2 v i peak oxi hóa COads t i 0.64 VNHE, th p h n đáng k so v i xúc tác Pt NPs/C (0.78 VNHE) Sau 5000 vòng quét

th tu n hoàn, s suy gi m ho t tính c a v t li u xúc tác 1D Pt3Co NWs/Ti0.7W0.3O2 là 10.68%, th p h n 4.18 l n so v i xúc tác Pt NPs/C (44.66%), đi u này ch ra đ b n xúc tác t t c a v t li u 1D Pt3CoNWs/Ti0.7W0.3O2 S c i thi n này có th là do (i) u

đi m c a c u trúc nano 1D v i nhi u v trí ho t hóa thúc đ y quá trình oxi hóa các phân methanol và ethanol; (ii) s xu t hi n c a Co thúc đ y vi c oxi hóa các s n ph m trung gian trong su t quá trình oxi hóa nhiên li u trên b m t Pt, d n t i t ng kh n ng ch ng

ng đ CO c a v t li u xúc tác t ng h p; (iii) hi u ng đi n t và liên h p gi a Pt3Co NWs và v t li u n n Ti0.7W0.3O2 H ng nghiên c u này có th m ra m t cách ti p c n

hi u qu đ nâng cao hi u qu xúc tác c a v t li u xúc tác đi n hóa v i s gi m l ng

Pt s d ng cho l nh v c chuy n hóa n ng l ng đi n hóa

ACKNOWLEDGEMENTS First of all, I would like to express my deepest gratitude to my advisors, Assoc Prof

Dr Van Thi Thanh Ho, and Assoc Prof Dr Son Truong Nguyen for suggesting the problem, supervising the work, and being a potential source of inspiration at each stage

of this dissertation research work I would like to express my deep thankfulness to Prof Nam Thanh Son Phan supported me during this dissertation research work at the University of Technology – Viet Nam University HCMC

I would like to give deep thanks to Dr Tai Thien Huynh for the collaboration during

4 years of working together His enthusiasm and support are highly appreciated

I would like to thank you for the support of the Faculty of Chemical Engineering – University of Technology – Viet Nam University HCMC, the MANAR Laboratory – Faculty of Chemical Engineering – University of Technology – Viet Nam University HCMC, the Physical Chemistry Laboratory – Ho Chi Minh City University of Natural Resources and Environment, the Applied Physical Chemistry Laboratory – University

of Science – Viet Nam University HCMC, and the Key Laboratory of Polymer and Composite Materials – University of Technology – Viet Nam University HCMC

My special thanks to my parents, and my girlfriend for understanding, encouragement, and consistent support throughout my dissertation journey Without their enthusiastic support, I could not complete my research

Finally, I acknowledge The Young Innovative Science and Technology Incubation Program, managed by Youth Promotion Science and Technology Center, Hochiminh Communist Youth Union, HCMC, Vietnam (Project No 10/2018/H -KHCN-V ), and Ph.D Scholarship Programme of Vingroup Innovation Foundation (VINIF) (No VINIF.2019.TS.22, VINIF.2020.TS.108 and VINIF.2021.TS.016) and University Scholarship of VNU – HCMC, 2019 for financial support

I sincerely thank you all!

TABLE OF CONTENTS

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF SYMBOLS AND ABBREVIATIONS xix

MOTIVATION OF RESEARCH 1

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW 2

1.1 Direct alcohol fuel cells (DAFCs) 2

1.1.1 Overview of DAFC technologies 2

1.1.2 Research history of direct alcohol fuel cells 3

1.2 Alcohol electro-oxidation on the Pt nanocatalysts 7

1.2.1 Methanol electro-oxidation reaction (MOR) on the Pt surface 7

1.2.2 Ethanol electro-oxidation reaction (EOR) on the Pt surface 9

1.3 Challenges of Pt-based catalyst in direct alcohol fuel cells 11

1.3.1 CO poisoning 11

1.3.2 Carbon corrosion 12

1.3.3 Platinum dissolution and growth 14

1.4 Non-carbon support for Pt-based electrocatalyst 16

1.4.1 Advantages and challenges of non-carbon nanosupport for DAFCs 16

1.4.2 State-of-the-art M-doped TiO2 support for alcohol electro-oxidation (AOR) 17

1.5 Tungsten-doped TiO2 nanosupport for direct alcohol fuel cells 19

1.6 One-dimensional (1D) Pt-based catalysts for the alcohol electro-oxidation 20

1.6.1 Advantages and challenges of 1D Pt-based nanostructures 21

1.6.2 Preparation of 1D Pt-based electrocatalysts 22

1.6.3 State-of-the-art 1D Pt-based electrocatalysts 24

1.7 Strategy and research objectives 26

CHAPTER 2 MATERIAL AND EXPERIMENTS 29

2.1 Chemicals 29

2.2 Experimental Sections 29

2.2.1 Fabrication of non-carbon TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports 29

2.2.2 Fabrication of 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) nanocatalysts 30

2.2.3 Fabrication of 1D Pt nanowires (NWs) on the Ti0.7W0.3O2 support 31

2.2.4 Preparation of 1D-bimetallic Pt3Co NWs on the Ti0.7W0.3O2 support 32

2.3 Physical Characterizations 33

2.3.1 X-ray diffraction (XRD) 33

2.3.2 X-ray photoelectron spectroscopy (XPS) 33

2.3.3 X-ray fluorescence (XRF) 34

2.3.4 Transmission electron microscopy (TEM) 34

2.3.5 Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDX) mapping measurement 34

2.3.6 Brunauer-Emmett-Teller (BET) method 35

2.3.7 Electrical conductivity measurement 35

2.4 Electrochemical Characterizations 35

2.4.1 Preparation of electrocatalytic ink 35

2.4.2 Electrochemical test 36

2.4.3 Cyclic voltammetry (CV) test 37

2.4.4 Alcohol electro-oxidation reaction (MOR, EOR) 38

2.4.5 CO-stripping voltammetry test 39

2.4.6 Accelerated durability test (ADT) 39

2.4.7 Chronoamperometry measurement 40

= 0.5; 0.6; 0.7; 0.8) NANOMATERIALS AS ROBUST NON-CARBON SUPPORTS FOR PLATINUM NANOPARTICLES IN DIRECT ETHANOL

FUEL CELLS 41

3.1 Characterization of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports 41

3.1.1 Effect of differential Ti: W ratio on nanostructures 41

3.1.2 Effect of differential Ti: W ratio on particle size and morphology 42

3.1.3 Effect of differential Ti: W ratio on the surface area 43

3.1.4 Effect of differential Ti: W ratio on the electrical conductivity 45

3.2 Characterization of 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) catalysts 47

3.3 Electrochemical characterization of electrocatalysts 50

3.4 Conclusion 58

CHAPTER 4 ONE-DIMENSIONAL Pt NANOWIRES ON Ti0.7W0.3O2 SUPPORT WITH EFFECTIVE ELECTRO-ACTIVITY FOR ALCOHOL ELECTRO-OXIDATION 59

4.1 Formation of the 1D Pt nanowires on Ti0.7W0.3O2 nanoparticles 59

4.1.1 Effect of reduction times on the growth of 1D Pt nanowires 59

4.1.2 Effect of loading amount of Pt on the growth of 1D Pt nanowires 61

4.1.3 A proposed mechanism for the formation of 1D Pt NWs/Ti0.7W0.3O2 62

4.2 Characterization of 1D Pt NWs/Ti0.7W0.3O2 catalyst 63

4.3 Electrochemical characterization of 1D Pt NWs/Ti0.7W0.3O2 catalyst 66

4.4 Conclusion 79

CHAPTER 5 SUPERIOR CO-TOLERANCE AND STABILITY FOR ALCOHOL ELECTRO-OXIDATION REACTION OF 1D-BIMETALLIC PLATINUM-COBALT NANOWIRES ON Ti0.7W0.3O2 NANOMATERIAL 80

5.1 Characterization of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst 80

5.2 Electrochemical properties of 1D Pt3Co NWs/Ti0.7W0.3O2 for MOR 82

5.3 Electrochemical properties of 1D Pt3Co NWs/Ti0.7W0.3O2 for EOR 87

5.4 Conclusion 91

CONCLUSIONS AND SCIENTIFIC CONTRIBUTION 92

LIST OF PUBLICATIONS 94

LIST OF CONFERENCES 95

LIST OF RESEARCH PROJECTS 96

REFERENCES 97

Appendix A 112

Appendix B 120

LIST OF TABLES

Table 1 1 Thermodynamic data associated with the electrochemical oxidation of some

fuels (under standard conditions) 2

Table 1 2 Products of direct alcohol fuel cells 6

Table 2 1 Summary of reaction temperatures for preparing W-doped TiO2 supports 30

Table 2 2 Summary of reaction times for preparing W-doped TiO2 supports 30

Table 2 3 Summary of Ti: W ratios for preparing W-doped TiO2 supports 30

Table 2 4 Summary of reaction times for fabricating 1D Pt NWs/Ti0.7W0.3O2 32

Table 2 5 Summary of Pt loadings for fabricating 1D Pt NWs/Ti0.7W0.3O2 32

Table 3 1 Summary of characterizations of non-carbon nanomaterials 46

Table 3 2 Comparison of the EOR performance of Pt-based NPs catalysts 54

Table 3 3 Comparison of EOR stability of catalysts after 5000 cycling test 56

Table 4 1 Binding energies of Pt in electrocatalysts 66

Table 4 2 Comparison of MOR performance of 1D Pt NWs/Ti0.7W0.3O2 catalyst 70

Table 4 3 Comparison of COads oxidation of 1D Pt NWs/Ti0.7W0.3O2 catalyst 72

Table 4 4 Comparison of MOR stability of catalysts after 5000 cycling test 73

Table 4 5 Comparison of EOR performance of 1D Pt NWs/Ti0.7W0.3O2 catalyst 75

Table 4 6 Comparison of EOR stability of catalysts after 5000 cycling test 77

Table 5 1 Comparison of MOR performance of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst 85

Table 5 2 Comparison of COads oxidation of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst 87

Table 5 3 Comparison of EOR performance of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst 89

LIST OF FIGURES

Figure 1 1 Scheme of direct alcohol fuel cells (DAFCs) 3

Figure 1 2 (a) Scheme of the first alkaline direct methanol fuel cells built by Kordesch and Marko; and (b) part of a 60-W methanol-air battery with cylindrical air diffusion electrodes built by Boveri et al 4

Figure 1 3 Proposed mechanism of parallel pathways for the methanol oxidation on pure Pt’s surface in acidic media 7

Figure 1 4 Proposed mechanism for the ethanol electro-oxidation on Pt surface in acidic medium (all species with colored filling were detected either by IR reflectance spectroscopy or by chromatographic analysis) 9

Figure 1 5 Electrosorption of methanol in an acidic medium 11

Figure 1 6 (a) Formation of radicals by the reaction of Pt, O2, and H2O; (b) carbon corrosion in the presence of Pt, O2, and H2O 13

Figure 1 7 Carbon corrosion in the absence of Pt 14

Figure 1 8 Proposed mechanisms for Pt NP instability in fuel cells 15

Figure 1 9 The most stable substance under fuel cell cathode conditions at 80 oC 17

Figure 1 10 Effect of dopants in Pt/M-doped TiO2 (M = V, Cr, and Nb) catalysts on the ORR performance 18

Figure 1 11 A summary of the kinds of quasi-one-dimensional nanostructures 21

Figure 1 12 Our motivation and approach to enhance AOR performance 27

Figure 2 1 The schematic illustration for fabricating W-doped TiO2 supports 29

Figure 2 2 The schematic illustration for preparation of the 20 wt% Pt/TixW1-xO2 31

Figure 2 3 The schematic illustration of the synthesis of 1D Pt NWs/Ti0.7W0.3O2 32 Figure 2 4 Cyclic voltammogram curves of 20 wt% Pt/C (E-TEK) catalyst in 0.5 M

Figure 2 5 Cyclic voltammogram curves of 20 wt% Pt/C (E-TEK) catalyst in 10 v/v%

CH3OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 39Figure 3 1 XRD patterns of TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports in the 2range from 20o to 80o at a step size of 0.02o 42Figure 3 2 XRD patterns of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports in the 2range from 22o to 32o at a step size of 0.02o 42Figure 3 3 TEM images of (a) undoped TiO2, (b) Ti0.8W0.2O2; (c) Ti0.7W0.3O2; (d)

Ti0.6W0.4O2; and (e) Ti0.5W0.5O2 supports 43Figure 3 4 Comparison of surface area of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports with reported non-carbon nanomaterials 44Figure 3 5 Electrical conductivity of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) supports 45Figure 3 6 X-ray diffraction (XRD) patterns of the 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) electrocatalyst in the 2range from 20o to 80o at a step size of 0.02o 48Figure 3 7 TEM images of (a) Pt/Ti0.8W0.2O2, (b) Pt/Ti0.7W0.3O2, and (c) Pt/Ti0.6W0.4O2 49Figure 3 8 High-resolution Pt 4f spectrum of (a) Pt/Ti0.8W0.2O2, (b) Pt/Ti0.7W0.3O2, (c) Pt/Ti0.6W0.4O2 and (d) Pt/C (E-TEK) catalysts 50Figure 3 9 Cyclic voltammograms of different electrocatalysts in N2-saturated 0.5 M

H2SO4 aqueous solution at a scan rate of 50 mV s-1 51Figure 3 10 Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%

C2H5OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1 52Figure 3 11 Onset potential of different catalysts in N2-saturated 10 v/v% C2H5OH/0.5

M H2SO4 solution at a scan rate of 50 mV s-1 53Figure 3 12 The mass activity of catalysts in N2-saturated 10v/v% C2H5OH/0.5 M

H2SO4 solution at a scan rate of 50 mV s-1 53Figure 3 13 Cyclic voltammograms of catalysts before and after 5000 cycling test in

N2-saturated 10v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 55Figure 3 14 Chronoamperogams of catalysts in N2-saturated 10 v/v% C2H5OH/0.5 M

H2SO4 aqueous solution at the fixed potential of 0.7 V for 7200s 57Figure 3 15 Effect of the tungsten doping content on electrocatalytic performance of Pt/TixW1-xO2 catalysts for ethanol electro-oxidation reaction 58Figure 4 1 XRD patterns of 1D Pt NWs/Ti0.7W0.3O2 catalyst with the reaction time (60 hours; 72 hours, and 84 hours) in the 2range from 20o to 80o at a step size of 0.02o 59Figure 4 2 TEM images of the 1D Pt NWs/Ti0.7W0.3O2 catalysts at different reduction times of (a) 60 hours, (b) 72 hours, and (c) 84 hours 60Figure 4 3 XRD patterns of 1D Pt NWs/Ti0.7W0.3O2 catalyst with different Pt amounts (40 wt%, 50 wt%, and 60 wt%) in the 2range in 20o-80o at a step size of 0.02o 61Figure 4 4 TEM images of the 1D Pt NWs/Ti0.7W0.3O2 catalysts at the different Pt loading of (a) 40 wt%, (b) 50 wt%, and (c) 60 wt% 62Figure 4 5 XRD patterns of the 1D Pt NWs/Ti0.7W0.3O2 catalyst in the 2range from

20o to 80o at a step size of 0.02o 63Figure 4 6 (a, b) TEM images, (c) HR-TEM image, and (d) Energy-dispersive X-ray (EDX) spectroscopy of the as-obtained 1D Pt NWs/Ti0.7W0.3O2 electrocatalysts 64Figure 4 7 High-resolution of Pt 4f spectrum of (a) 1D Pt NWs/Ti0.7W0.3O2, (b) Pt NWs/C, (c) Pt NPs/C, (d) Comparison of binding energies of Pt 4f spectrum in catalysts 65Figure 4 8 Cyclic voltammograms of different catalysts in N2-saturated 0.5 M H2SO4aqueous solution at a scan rate of 50 mV s-1 66Figure 4 9 Cyclic voltammograms before and after 5000-cycling ADT of different electrocatalysts in N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s-1 67Figure 4 10 Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%

CH OH/0.5 M H SO solution at a scan rate of 50 mV s-1 68

Figure 4 11 Onset potential of different catalysts in N2-saturated 10 v/v% CH3OH/0.5

M H2SO4 solution at a scan rate of 50 mV s-1 69Figure 4 12 The mass activity of different catalysts in N2-saturated 10 v/v%

CH3OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1 70Figure 4 13 CO-stripping cyclic voltammograms of differential catalysts in 0.5 M

H2SO4 aqueous solution at a scan rate of 50 mV s-1 71Figure 4 14 Cyclic voltammograms of catalysts before and after 5000 cycling test in

N2-saturated 10v/v% CH3OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 72Figure 4 15 Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%

C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 74Figure 4 16 A comparison of the mass activity of different catalysts in N2-saturated 10 v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 74Figure 4 17 Cyclic voltammograms of catalysts before and after 5000 cycling test in

N2-saturated 10v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 76Figure 4 18 Chronoamperogams of catalysts in N2-saturated 10 v/v% C2H5OH/0.5 M

H2SO4 aqueous solution at the fixed potential of 0.7 V for 7200s 78Figure 4 19 Schematic illustration for improvement of 1D Pt NWs/Ti0.7W0.3O2catalysts toward methanol and ethanol electrochemical oxidation 79

Figure 5 1 XRD patterns of the 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst in the 2range from 20o to 80o at a step size of 0.02o 81Figure 5 2 (a, b) TEM and (c) HR-TEM images of the Pt3Co NWs/Ti0.7W0.3O2 catalyst 81Figure 5 3 X-ray fluorescence (XRF) spectroscopy of 1D Pt3Co NWs/Ti0.7W0.3O2 82Figure 5 4 Cyclic voltammograms of different catalysts in N2-saturated 0.5 M H2SO4aqueous solution at a scan rate of 50 mV s-1 83Figure 5 5 Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%

CH3OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1 84

Figure 5 6 The mass activity of different catalysts in N2-saturated 10 v/v% CH3OH/0.5

M H2SO4 aqueous solution at a scan rate of 50 mV s-1 85Figure 5 7 CO-stripping cyclic voltammograms of differential catalysts in 0.5 M

H2SO4 aqueous solution at a scan rate of 50 mV s-1 86Figure 5 8 Cyclic voltammograms of catalysts in N2-saturated 10 v/v% C2H5OH/0.5

M H2SO4 solution at a scan rate of 50 mV s-1 88Figure 5 9 The Mass activity of different catalysts in N2-saturated 10 v/v%

C2H5OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1 88Figure 5 10 Cyclic voltammograms of catalysts before and after 5000 cycling test in

N2-saturated 10v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1 90Figure 5 11 Chronoamperogams of the as-obtained catalysts in N2-purged 10 v/v %

C2H5OH/0.5 M H2SO4 solution at 0.70 V for 7200 s 91

LIST OF SYMBOLS AND ABBREVIATIONS

AOR Alcohol electro-oxidation reaction

ADT Accelerated durability test

DAFCs Direct alcohol fuel cells

DEFCs Direct ethanol fuel cells

DMFCs Direct methanol fuel cells

ECSA Electrochemical surface area

EDX Energy dispersive spectroscopy

EOR Ethanol electro-oxidation reaction

HR-TEM High-resolution transmission electron microscopy JCPDS Joint Committee on Powder Diffraction Standards MOR Methanol electro-oxidation reaction

SMSI Strong metal-support interaction

RHE Reversible hydrogen electrode

TEM Transmission electron microscopy

XPS X-ray photoelectron spectroscopy

MOTIVATION OF RESEARCH

Nowadays, the worldwide environment has been getting worse day by day because

of the emission of various harmful pollutants into the environment from burning traditional fossil fuels In addition, fossil fuel resource is non-renewable and will be exhausted in the next few decades, therefore, finding out green and renewable power sources have been sparked interest as future alternatives Since William R Grove discovered that electricity could be generated directly from gaseous hydrogen and oxygen, a fuel cell has been extensively developed into many different types such as alkaline fuel cell, phosphoric acid, molten carbonate, solid oxide, and proton-exchange membrane fuel cells Of the multitude of fuel cells available, direct alcohol fuel cells (DAFCs) use liquid and low-cost renewable fuels that have increasingly become important for portable and transportation applications owing to their relatively simple infrastructure, portability, operation cost, and facile storage, and conveyance

In DAFC systems, nanocatalysts play a significantly important role in the half-cell oxidation and reduction processes at anode and cathode electrodes, respectively Up to now, zero-dimensional (0D) Pt nanoparticles (NPs) on carbon support are currently utilized as state-of-the-art DAFC catalysts, but the slow anodic oxidation kinetics and CO-poisoning effect of Pt are large limitations for commercializing DAFCs Additionally, carbon corrosion can also cause loss and agglomeration of Pt NPs, resulting in declining fuel cell performance Recently, using carbon-free support and tuning the metal catalyst structure have emerged as efficient approaches to address the problems of catalysts

For these mentioned reasons, we propose the study entitled: “The Multifunctional

TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) Support for Platinum to enhance the activity and CO-tolerance of Direct Alcohol Fuel Cells” This efficient approach can improve electrocatalytic activity and stability of anodic catalysts, promoting large-scale DAFC applications and reducing the dependence on fossil fuels

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW

1.1 Direct alcohol fuel cells (DAFCs)

1.1.1 Overview of DAFC technologies

A fuel cell is an electrochemical device that continuously and directly converts the

chemical energy of externally supplied fuel into electrical energy Hydrogen is the most

common fuel due to its fast oxidation kinetic and high efficiency of hydrogen/oxygen

fuel cells, which was first explored by William Grove in 1839 [1-3] However, hydrogen

is not a primary fuel, meaning it has to be produced from other sources; namely, natural

gas reforming, oil or coal gasification, and water electrolysis [1] In addition, the

production and distribution difficulty of clean hydrogen are challenges for the use of

hydrogen-feel fuel cells

Table 1 1 Thermodynamic data associated with the electrochemical oxidation of some

fuels (under standard conditions) [4]

o

(kJ mol-1)

Standard theoretical potential, Eo

Energy density (Wh L-1)

- Ho(kJ mol-1)

Reversible energy efficiency Hydrogen 0 0.000 180 (@1000 psi, 25 oC) 285.8 0.830

In the current era, alcohols (e.g., methanol and ethanol) emerge as a promising

alternative because they are liquid under ambient temperature and pressure, making

their facile storage and distribution facile and safe [1, 3, 5] Furthermore, alcohols have

a high energy density (4000 – 7000 Wh L-1) which is more similar to those of

hydrocarbons and gasoline (9800 Wh L-1) [4], as listed in Table 1.1 In the last decades,

there has been extensive research on direct methanol fuel cells (DMFCs) for portable

power applications at low to moderate temperatures because methanol has the simplest alcohol structure, consisting of only one carbon atom, and thus it is easier to oxidize at anode electrode and shows higher selectivity towards CO2 formation during oxidation process compared to other alcohol molecules (Figure 1 1) However, the main source

of methanol world production is 90% from natural gas [6]

Figure 1 1 Scheme of direct alcohol fuel cells (DAFCs)

Among the other alcohols, ethanol is an attractive alternative to methanol as a fuel for fuel cells because ethanol is a renewable fuel and can be produced in large quantities from farm products and biomass Ethanol is non-toxic and its energy density (6280 Wh

L-1) is higher than that of methanol (4820 Wh L-1) The problem with ethanol as a fuel

is that the complete oxidation of ethanol molecules requires cleavage of the C-C bond

in it, which is difficult on the state-of-the-art catalyst at temperatures lower than 100

oC This typically results in incomplete oxidation of ethanol, which decreases the fuel cell efficiency and can produce toxic by-products or electrode deactivation

1.1.2 Research history of direct alcohol fuel cells

The detailed research of the electrochemical oxidation process of methanol and other organic compounds at platinum anodes in aqueous alkaline electrolytes was firstly reported by Muller in 1922 [7] By laboratory investigations, alkaline direct methanol fuel cells (ADMFCs) were firstly built by Kordesch’s group in 1951 However, its main trouble was cross-leakage of the methanol to cathode electrode harming noble metal catalyst Based on ADMFCs, Boveri et al [8] developed a 6 V, 10 A battery for flashing sea buoy that consisted of ten cylindrical cells with each cell having 18 pairs of electrodes connected in parallel, as shown in Figure 1 2

Figure 1 2 (a) Scheme of the first alkaline direct methanol fuel cells built by Kordesch and Marko [9]; and (b) part of a 60-W methanol-air battery with cylindrical air diffusion electrodes built by Boveri et al [8]

During the 1960s, Exxon-Alsthom in France designed alkaline and buffer electrolyte DMFCs; however, carbonation by complete methanol oxidation into CO2 was a problem, which forced pull out the research in the late 1970s [10] Therefore, since the late 1960s and 1970s, pioneering studies on the MOR were performed in acidic media and explored that MOR kinetic in acidic media was slower than in alkaline media [11]

In the late 1950s and early 1980s, DMFCs with concentrated sulfuric acid were developed by Shell Research Center (England) and Hitachi Research Laboratories

(Japan) focused on their application to road transportation [10] The first approach was

to utilize new thin electrodes in alkaline media and then operated a 5 kW stack DMFCs After the oil industry recovered from the 1973 crisis, the development of DMFCs was discontinued by a decrease in oil prices, resulting in cost targets for DMFCs even further In the late 1970s and early 1980s, other acid DMFCs have been investigated for different applications such as military communication systems (US Army) and electronic wheelchairs (Royal Institute of Technology, Stockholm) [1] During the past decades, DMFCs and DEFCs emerge as potential energy conversion technologies in portable and transportation applications (Table 1.2)

Table 1 2 Products of direct alcohol fuel cells [12]

Transportation

Industrial Military

Telecommunication

DMFC Dynario TM (Battery charger) Toshiba The maximum power output of 2 W with a single injection of 14 mL of concentrated methanol

Other

DEFC Bio-energy discovery kit Horizon Fuel Cell Technology A working demonstration of the conversion of ethanol to electrical energy DMFC Mobile audio player Toshiba & Hitachi 100 mW capacity for 35 hours of player usage with a single 3.5 mL highly concentrated methanol DMFC Mobile audio player Toshiba & Hitachi 300 mW capacity for 60 hours of player usage with a single 10 mL highly concentrated methanol

1.2 Alcohol electro-oxidation on the Pt nanocatalysts

1.2.1 Methanol electro-oxidation reaction (MOR) on the Pt surface

The methanol oxidation process on Pt surface in acidic media has been studied by many groups and a commonly accepted mechanism is shown in Figure 1 3 [13]

Figure 1 3 Proposed mechanism of parallel pathways for the methanol oxidation on pure Pt’s surface in acidic media [13]

All pathways in Figure 1 3 can be divided into two steps including methanol dehydrogenation, stripping, and oxidation of intermediate species A detail of the MOR mechanism on Pt catalyst is demonstrated by A Hamnett [13-15]:

 Methanol dehydrogenation to generate adsorbed intermediates

CH3OH + Pt  Pt-CH2OH + H+ + e- Pt-CH2OH + Pt  Pt2-CHOH + H+ + e-

Pt2-CHOH + Pt  Pt3-CHO + H+ + e-

Pt3CHO  2Pt + Pt-CO + H+ + e

-(1.1) (1.2) (1.3) (1.4) The electronic and crystal structures of Pt play a key role in determining the methanol dehydrogenation process By controlling the potential, Kang et al [16] reported that the downshift of the Pt d-band center reduced dehydrogenation activity on the Pt surface Another result of Spendelow et al [17, 18] demonstrated different roles of Pt surface

CO oxidation but do not affect methanol dehydrogenation Meanwhile, large islands, like step-type adsorption sites, promote both methanol dehydrogenation and CO oxidation

In the methanol dehydrogenation process, step-type adsorption sites seem to be more active by the geometric arrangement of surface atoms In contrast, the electronic perturbations at kinks are responsible for high CO adsorption activity [13]

 Water degradation to produce oxygenated species

 Formation of carbon dioxide (CO2) to complete the overall reaction

HCHO + H2O  CO2 + 4H+ + 4e-

HCOOH  CO2 + 2H+ + 2ePt-COOH  Pt + CO2 + H+ + e-Pt-CO + Pt-OH  2Pt + CO2 + H+ + e-

-(1.10) (1.11) (1.12) (1.13) All above reactions are possible in MOR, and it is essential to determine carbonaceous species by Fourier-transform infrared spectroscopy and differential electrochemical mass spectroscopy The MOR is an irreversible reaction with six electrons transfer, and its mechanism is still hard to establish even with well-identified intermediate species [13]

1.2.2 Ethanol electro-oxidation reaction (EOR) on the Pt surface

Similar to MOR, the EOR on the Pt surface undergoes two steps including (i) adsorption and dehydrogenation of ethanol molecules; (ii) stripping and oxidation of intermediate species However, the EOR is more complicated because ethanol contains two carbon atoms The EOR mechanism on Pt surface has been studied by different methods and a commonly accepted mechanism is shown in Figure 1 4 [19-22]

Figure 1 4 Proposed mechanism for the ethanol electro-oxidation on Pt surface in acidic medium (all species with colored filling were detected either by IR reflectance spectroscopy or by chromatographic analysis).[23]

The proposed mechanism of EOR on Pt surface in acidic electrolyte including the first step of ethanol adsorption via an O-adsorption or a C-adsorption resulting in

Pt + CH3CH2OH  Pt-(OCH2CH3)ads + H+ + e-

Pt + CH3CH2OH  Pt-(CHOHCH3)ads + H+ + e-

(1.14) (1.15) The AAL formation at a potential lower than 0.6 VRHE has been reported and acetic acid was generated as intermediate of this reaction according to proposed mechanism of Vigier et al [19]:

E < 0.6 VRHE

Pt-(OCH2CH3)ads  Pt + CHOCH3 + H+ + e-

Pt-(CHOHCH3)ads  Pt + CHOCH3 + H+ + e-

(1.16) (1.17) The next step is the formation of Pt-COCH3 species [19]:

E < 0.4 VRHE

Pt + CHOCH3  Pt-(COCH3)ads + H+ + e (1.18) According to the result of Vigier et al [19], CO species can adsorb on Pt surface at potentials from 0.3 VRHE; meanwhile, the study carried out by Iwasita et al [20] found

CH4 traces at a potential lower than 0.4 VRHE

E > 0.6 VRHE

Pt + H2O  Pt-(OH)ads + H+ + e- (1.21)

Pt-(CO)ads + Pt-(OH)ads  2Pt + CO2 + H+ + e- Pt-(CHOCH3)ads + Pt-(OH)ads  2Pt + CH3COOH + H+ + e-

(1.22) (1.23)

1.3 Challenges of Pt-based catalyst in direct alcohol fuel cells

1.3.1 CO poisoning

Taking MOR as an example, different intermediate species are produced and affect directly the electrocatalytic performance of Pt-based catalysts Among all adsorbed carbonaceous intermediates, COads are considered the most harmful species to active sites

of catalyst by their strong adsorption, assigning to charge donation from a lone-pair orbital of adsorbed CO species to 5d orbital of Pt atoms and a black donation from 5d orbital of Pt atoms to the anti-bonding 2*orbital of molecular CO [13, 24] The CO adsorption on the Pt surface is usually carried out by two pathways like liner and bridge, depending on Pt facets [13] The subsequent methanol chemisorption on Pt sites is suppressed, thereby the severely decayed performance of catalysts (Figure 1 5)

Figure 1 5 Electrosorption of methanol in an acidic medium [25]

At potential below 0.45 VRHE, the surface of Pt is easily poisoned by a layer of strongly bonded COads species and further chemisorption of methanol molecules cannot proceed until the surface-bound COads are oxidized from the Pt surface At potential below 0.45

VRHE, this process occurs at a slow rate and thus Pt surface remains poisoned throughout

and constantly oxidized into CO2 in the range of 0.5 VRHE ~ 0.7 VRHE The Pt active sites are almost free of COads species at a potential above 0.7 VRHE by the COads oxidation according to the reaction (1.24) Under operation conditions of DMFCs, anode potential

is much lower than 0.5 VRHE, thereby the poisoning effect seriously affects electrocatalytic activity and durability of catalysts For alkaline electrolytes, the CO poisoning effect is less severe because the binding of COads is not as strong as in acidic media by the adsorption of OH- [26] Nonetheless, the CO poisoning on the Pt surface still exists in an alkaline environment This results in numerous research efforts to design alternative nanomaterials, which can oxidize methanol molecules at a lower potential

Pt-COads + Pt-OHads  2Pt + CO2 + H+ + e- (1.24) 1.3.2 Carbon corrosion

Carbon materials are widely used as catalyst support for Pt-based catalysts in fuel cells

by their large surface area, high electrical conductivity, and affordability [27] However, carbon corrosion is a major cause that declines catalytic performance under long-term operation [27, 28] Carbon corrosion in acidic media according to the following reaction:

In general, the electrochemical corrosion of carbon support is slow and negligible at

a potential below 1.1 VRHE, however, Pt presence is believed to reduce the potential of carbon oxidation and accelerate carbon corrosion [23] At temperatures from 50 oC to 90

oC and low pH (< 1), carbon atoms can react with oxygen atoms and/or water molecules

to form gaseous products like CO and CO2, which leave the cell with oxygen and vapor stream [26] During the startup/shutdown of fuel cells, the cathode potential can reach as high as 1.5 VRHE, which drastically accelerates carbon corrosion [26] Cai’s group [29] studied the effect of oxygen and water concentration on two commercial Pt catalysts on carbon supports with different BET areas (Pt/Vulcan and Pt/HSC) This result reported different mechanisms of carbon corrosion with and without Pt In the presence of Pt, high reactive OH and OOH radicals are formed by the reaction of Pt with oxygen and water (Figure 1 6(a)) These radicals can attack carbon to form surface oxygen groups, which can then decompose at high temperatures to produce CO and/or CO2 (Figure 1 6(b))

Figure 1 6 (a) Formation of radicals by the reaction of Pt, O2, and H2O; (b) carbon corrosion in the presence of Pt, O2, and H2O [29]

In the absence of Pt, oxygen can react with carbon through the formation of radicals because carbon has unpaired electrons [29, 30], as described in Figure 1 7

Figure 1 7 Carbon corrosion in the absence of Pt [29]

1.3.3 Platinum dissolution and growth

Up to now, Pt-based NPs loading on carbon supports is widely utilized as catalysts for both anode and cathode electrodes in DAFCs Electrocatalysts with particle size in the nanometre range exhibit large electrochemical surface area (ECSA) and also have thermodynamic instability because of a high specific surface free energy [23], resulting

in the further growth of Pt NPs The possible growth mechanism of Pt NPs in Figure 1

8 includes local coalescence of agglomerated NPs, which is the agglomeration of nonadjacent crystallites by migration and subsequent ripening of Pt NPs [31, 32], making the ECSA loss under long-term operation conditions

Figure 1 8 Proposed mechanisms for Pt NP instability in fuel cells [33]

Generally, four processes are shown in Figure 1 8 that are relevant to the ECSA loss

of Pt-based catalysts in fuel cell electrodes, including:

1 Ostwald ripening based coarsening of individual Pt NPs on the carbon surface, which may involve Pt dissolution from small NPs, diffusion of soluble Pt species from small to large NPs in ionomer phase, and redeposition or reduction of Pt species onto large NPs (Figure 1 8(a)), which is analogous to Ostwald ripening process [33-35]

2 Pt crystal migration and coalescence are related to the motion and coalescence

of Pt NPs in which they meet on carbon’s surface (Figure 1 8 (b)) In the absence of electrolyte, crystallite migration and coalescence are observed in gas-phase sintering studies of Pt/C catalyst but are insignificant at below 500

oC temperature in the gas phase [36]

3 A detachment of Pt NPs from carbon support and agglomeration of Pt NPs is generally caused by carbon corrosion (Figure 1 8(c)) This phenomenon includes the activity loss of fuel cell electrodes, depending on cell voltage and nature of the interaction between Pt NPs and carbon support, as well as graphitization degree of the carbon support, and potentially other factors [33]

4 Dissolution and reprecipitation of platinum single crystals in the ionomer and membrane by the chemical reduction of soluble Pt species with hydrogen molecules, resulting in the activity loss [37, 38] (Figure 1 8(d))

1.4 Non-carbon support for Pt-based electrocatalyst

As aforementioned, the long-term durability of catalysts is one of the most limitations for commercializing DAFCs Carbon corrosion occurs at voltages above 0.9 VRHE, causing dissolution and agglomeration of Pt NPs, leading to a decrease in overall fuel cell efficiency Therefore, it is necessary to develop robust supports with high durability and strong interaction between them and metal NPs under electrochemical media 1.4.1 Advantages and challenges of non-carbon nanosupport for DAFCs

Metal oxides based on non-noble metals have been used widely in a variety range of fields due to their abundant sources, low cost, and environmental friendliness [39-41] In DAFCs, metal oxides have advantages such as (i) much higher corrosion resistance in the electrochemical environment compared to carbon support because metals in metal oxides exist in a high oxidation state and do not readily lose further electrons to be further oxidized [42]; (ii) strong interaction between them and metal NPs, which can prevent agglomeration of metal NPs and retain small metal particle sizes [40, 43]; (iii) a large number of hydroxyl groups on their surface that can act as co-catalysts for noble metal catalysts through bifunctional mechanism [43, 44] Due to their merits, metal oxides have been utilized as independent catalysts, co-catalysts, and supports for fuel cells in acidic

and alkaline electrolytes Sasaki et al [45] conducted thermochemical calculations to determine stable supports under fuel cell conditions, in which 1.0 VSHE potential; pH =

0, and 80 oC They indicated W, Ti, Sn, Nb, Ta, Sn, and Sb are stable as oxides, hydroxides, or metals under these conditions (Figure 1 9) However, small surface area and low electrical conductivity are disadvantages of metal oxides for fuel cells [46]

Figure 1 9 The most stable substance under fuel cell cathode conditions at 80 oC [45]

In fuel cells, several prerequisites need to be considered for choosing the suitable metal oxide supports including superior durability in the electrochemical media, high resistance against electrochemical corrosion and electrical conductivity, compatibility with the electrode, good proton conductivity, and large surface area [39, 40, 47]

Among these metal oxides, TiO2 has been attractive to a wide range of industries due

to its unique properties [47, 48] TiO2 can improve fuel cell performance either as support

or carrier or as membrane and as catalyst itself [47] The crystal structure of the rutile and anatase form of TiO2 makes it more thermodynamically stable resulting in high thermal and electrochemical stability of composite materials [47] In addition, the electronic interaction between TiO2 and metal NPs can enhance catalytic performance due to a decrease in carbonaceous adsorption [49-51] However, the low electrical conductivity of TiO2 is a major challenge [49, 52-54]

1.4.2 State-of-the-art M-doped TiO2 support for alcohol electro-oxidation (AOR)

only boosts the electrical conductivity of TiO2 but also enhances the performance of catalysts in fuel cells, which has been proven by experimental results [52, 54-58] and density functional theory (DFT) calculations [43, 55] For instance, Kim et al [56] fabricated various M-doped TiO2 (M = V, Cr, and Nb) by sol-gel and/or hydrothermal route to investigate the effect of doping on ORR activity of Pt/M-doped TiO2 catalysts The Pt loading on all M-doped TiO2 supports exhibited much higher ORR activity than the commercial Pt/C catalyst and the highest activity was achieved on the Pt/V-doped TiO2 catalyst The enhancement was explained due to electron transfer from M-doped TiO2 supports to Pt NPs and compressive strain at the Pt surface, inducing the downward shift of the Pt d-band center The excellent ORR stability of Pt/V-doped TiO2electrocatalyst resulted from an anchoring effect deriving from strong metal-support interaction (SMSI) The authors indicated that the introduction of dopants can cause compressive strain of Pt, resulting in enhanced ORR performance (Figure 1 10)

Figure 1 10 Effect of dopants in Pt/M-doped TiO2 (M = V, Cr, and Nb) catalysts on the ORR performance [56]

In 2019, Huynh et al [57] reported Ti0.7Ir0.3O2 nanomaterial as non-carbon support for

Pt catalyst for MOR in acidic media The Ti0.7Ir0.3O2 support was prepared by a one-step hydrothermal process without using a surfactant or further heat treatment The authors demonstrated that 20 wt% Pt/Ti0.7Ir0.3O2 catalyst was a promising catalyst to replace the commercial Pt/C (E-TEK) catalyst owing to its high activity and durability with low onset potential (0.1 VNHE), and high current density (21.69 mA cm-2) The improvement

was explained by electronic transfer from Ti0.7Ir0.3O2 to Pt NPs, leading to a modification

of the surface electronic structure of Pt catalyst However, the high cost of iridium metal

is a major limitation for commercializing DMFCs

In 2020, Noh et al [58] introduced Nb-doped TiO2 nanomaterial as catalyst support for Pt NPs towards ORR that was prepared by hydrothermal route using commercial TiO2 P25 (Degussa Co.) and niobium ethoxide (Nb(OCH2CH3)5) as precursors The Pt/Nb-doped TiO2 showed a remarkable enhancement of ORR kinetics compared to commercial Pt/C catalyst After the accelerated durability test, the high ORR stability was observed on Pt/Nb-doped TiO2 catalyst with a decline of 22% in initial mass activity, against a large decrease of 37% of Pt/C catalyst These improvements were due to an increase in electronic conductivity and SMSI between Nb-doped TiO2 and Pt NPs

In 2020, Fe-doped TiO2 nanomaterial was prepared by Ferreira’s group [59] through

a sol-gel process using titanium (IV) isopropoxide and iron nitrate as precursors The doped TiO2 nanomaterial was employed as catalyst support for Pt NPs towards EOR in acidic media The XPS results revealed a surface rich in metallic Pt and oxygen species (O2- species, OH- and O atoms) in the vicinity of oxygen vacancies, and SMSI between

Fe-Pt NPs and Fe-doped TiO2 Electrochemical outcomes indicated that catalytic activity of Pt/Fe-doped TiO2 catalyst was higher than Pt/TiO2 and Pt/C catalyst, attributing to synergic and electronic effects of compounds, Pt, and Fe-doped TiO2 support

1.5 Tungsten-doped TiO2 nanosupport for direct alcohol fuel cells

The W-doped TiO2 support for Pt NPs in PEMFCs was first introduced by Wang’s group [60] Electrochemical results indicated that Pt/Ti0.7W0.3O2 catalyst showed high CO-tolerance and catalytic activity for hydrogen oxidation reaction (HOR) compared to PtRu/C (E-TEK) catalyst In addition, results of differential electrochemical mass spectrometry demonstrated the low onset potential (<0.1 VRHE) of COads oxidation on Pt/Ti0.7W0.3O2 Another study by Subban et al [61] reported Ti0.7W0.3O2 nanomaterial was durable catalyst support that was much more stable to oxidative decomposition than black carbon Furthermore, no evidence of decomposition was observed on Ti0.7W0.3O2support when heated in Nafion solution for 3 weeks at 80 oC, indicating superior